Ion trap geometry

This chapter is organized as follows. First, the need for still further enhancement in the precision with which time is measured will be justified, and the concept of atomic clocks and their properties will be described in detail. Then, the properties required for such an atomic system suitable for time and frequency metrology will be developed as well as the conditions necessary to attain them, following schemes involving either ions or neutral atoms. For the utilization of ions in atomic clocks, the well-known technique of ion trapping is used. The next part of the discussion will be devoted, therefore, to both a panoramic view of the ion trap geometries used... 

Here z and M are charge and mass (in Daltons) of the trapped ions, respectively, /= Q/2jt is the frequency of the RF field (in Hz), ro is the inscribed radius of the trap expressed in cm, and Ura is the maximum kinetic energy of the ions (in eV units) that can be trapped. For a particular ion trap geometry (i.e., n, ro), Eq. (6) determines the minimum RF amplitude F in (at t] 0.3) that can be used to trap ions with transverse kinetic energy (expressed in eV). Equations (2), (3) and (5) then allow an evaluation of the minimum frequency required for trapping of ions with a particular M/z. 

Major ion trap developments have occurred over the last 15 years. In 1995, Bier and Syka patented the use of the mass selective instability scan from high-charge-capacity ion trap geometries such as the linear quadmpole ion trap (LQIT), toroidal trap (TQIT), curved or banana traps (CQIT), and elliptical traps (EQIT). The LQIT with radial ejection was eventually commercialized in 2002, and it is used as a stand-alone mass analyzer and has been combined with QMFs, Fourier transform ion cyclotron resonance (FT ICR), and the orbitrap to form hybrid instmments. The LQIT analyzer has eclipsed the conventional ... 

Principle. The cylindrical quadrupole ion trap is based on the same principle as the quadrupole mass filter, but the geometry is different (Fig. 2.16). The cylindrical QIT, or Paul trap, was developed almost simultaneously with the quadrupole mass filter [232, 233]. Recently, a variant of the theme has emerged, the linear quadrupole ion trap [236], which is a device built like a quadrupole mass filter with extra trapping end electrodes for the axial direction. Under stable conditions, ions moving around inside such traps will ideally continue to do that forever. 

The sensitivity, compactness, automation, and low prices of ion trap instruments made them very popular in biological MS P Limitations of ion traps include low resolution and mass accuracy at high m/z. In addition, in MS/MS mode, the lower end of the fragment mass range cannot be visualized. Recent developments in the linear geometry of ion traps are aimed at improving on those limitations. 

Mass analyzer based on rectilinear geometry ion trap... 

Fig. 10. Rectilinear geometry ion trap (RIT) assembly (a) and instrumentation (b) Reproduced with permission from Ouyang et al. [19]. Copyright 2004 American Chemical Society.

The operating principle of a quadrupole ion trap mass analyzer is similar to that of a standard quadmople but the geometry is different. A trap consists of three hyperbolic electrodes comprising a ring and two endcap electrodes. Applying voltages to these electrodes, results... 

Other mass spectrometers are equipped with three-dimensional ion traps of which the geometry is much different to the quadrupoles previously described. In an ion-trap, the ions are confined between three electrodes (one toroidal and two end-caps), whose particular shape appears to result from a sort of anamorphosis of the four-bar set-up of a classic quadrupole. As in the previous category they operate under the effect of a variable electric field (with or without a superimposed fixed field). Although they are, in appearance, physically simple devices, the fundamental principle of ion trap is complex. These ion trap detectors are sensitive, less costly than quadrupoles and compatible with different ionization techniques. The volume defined by the electrodes, named superior, inferior and annular, is simultaneously the ion source and the mass filter (Figure 16.11). These analysers are almost exclusively linked with a separative technique (GC/MS). 

The quadrupole ion trap (QIT) mass spectrometer consists of three hyperbolic-shaped electrodes arranged in a cylindrical geometry (see Fig. 2.14). By considering its axial geometry, we can move from the classical Cartesian coordinates to the polar ones. In fact, each point of the space inside the trap can be defined by the value of its axial (z) and radial (r) coordinates. If a potential U + Vcos cot is applied at the intermediate electrode and the two end caps are grounded, a mathematical treatment analogous to that done for quadrupole mass filter can be employed. In this case, a and q values can be defined again as... 

For the former approach, electrodynamic fields of intensity higher than the quadrupolar are employed. As summarized in Table 2.1, while a quadrupolar field increases linearly with the distance, for hexapoles it increases quadratically, and for octapolar it increases cubically. The last two conditions can be obtained by different electrode configurations (see Fig. 2.19), but components of higher fields can be simply obtained by varying the geometry of the ion trap, in particular the asymptotic cone... 

Figure 2.20. Higher order field components can be obtained by changing the geometry of a classical quadrupolar ion trap.

Ion-trap methods include Fourier transform ion cyclotron resonance MS (FTICR-MS) and quadrupole ion traps (QIT or Paul traps). In these methods, ions are produced in or transferred into a region with appropriate geometry walls and some combination of magnetic fields, DC potentials, and RF potentials that confine the ions on the timescale of seconds to days. FTICR-MS has been particularly popular for the study of organometallic ions in the gas phase. In FTICR-MS, the ions are confined by a magnetic field that constrains the ions to the center of the cell. 

Strife, R. J. KeUey, P.E. Weber-Grabau, M. Tandem mass spectrometry of prostaglandins A comparison of an ion trap and a reversed geometry sector instrument. Rapid Commun. 

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